here. Another two are:
Larry Niven's second, shorter, untitled future history which features the Smoke Ring, a breathable atmosphere in a gas torus around a neutron star;
ERB's Moon Trilogy which covers several generations and is a sequel to his Mars series as is Beyond The Farthest Star, set in a planetary system where several planets share an orbit and are linked by a breathable atmosphere so that an interplanetary crossing can be made by aircraft.
My challenge to Anderson if he were still alive - or maybe to Niven? - would be: rationalize ERB's absurd celestial mechanics. If the Smoke Ring is possible, can ERB's Poloda be made somehow plausible? Maybe celestial engineering was involved as in SM Stirling's Lords of Creation series where a habitable Mars and Venus are discovered but only because of ancient terraforming?
Although Poul Anderson's output was enormous, there are still works that we would like to see, like:
an indefinitely extended Time Patrol series and Technic History;
a continuation of his contributions to other sf series like maybe to ERB's.